Stage 1 patients are not eligible for Stage 2. In Stage 2, patients will be stratified by body weight, then randomly assigned to one of two arms. Arm A will receive the fixed-dose tablet twice daily for 4 weeks, then switch to the individual liquid formulations twice daily for 4 weeks. Arm B will receive the individual liquid formulations twice daily for 4 weeks, then switch to the fixed-dose tablet twice daily for 4 weeks. To encourage medication adherence, study staff will make home visits and phone calls to each patient's parent or guardian the first week of each treatment regimen. Medical history, documentation of direct observation of therapy (DOT), a physical exam, and urine collection will occur on Days 25 and 53. Also on Days 25 and 53, patients will be admitted to the hospital to ensure 100% medication adherence and to provide blood for PK studies.